The present application is based on Japanese priority application No.2000-067790 filed on Mar. 10, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to an optical signal processor based on a semiconductor device having quantum dots.
In the art of long-range optical telecommunication, it is practiced to provide optical signal repeaters in an optical fiber network system with a suitable interval so as to compensate for a decay or phase-shift of optical signals transmitted through the optical fiber network over a long distance.
Thus, a repeater carries out processing of repeating the optical signals, wherein such a repeating process includes extraction of clocks from the optical signals, re-modulation of the optical signals by using the clocks thus extracted, elimination of noise from the optical signals, and the like.
FIG. 1 shows the schematic construction of an optical signal repeater 10 of a wavelength-division multiplexed optical signal according to a related art for use in a large-capacity, long-range optical telecommunication system.
Referring to FIG. 1, the optical signal repeater 10 receives an incoming wavelength-division multiplexed optical signal containing a number of channels CH1-CHN having respective, mutually different wavelengths, and includes an optical demultiplexer 10A decomposes the incoming wavelength-division multiplexed optical signal into optical signal components of individual channels CH1-CHN. Further, the optical repeater 10 includes a plurality of repeating units 10B1-10BN respectively provided in correspondence to the channels CH1-CHN, wherein each of the repeating units 10B1-10BN carries out the repeating process of the optical signal component supplied thereto. Further, the optical repeater 10 includes an optical multiplexer 10C that synthesizes a wavelength-division multiplexed optical signal from the output optical signals of the repeating units 10B1-10BN.
FIG. 2 shows the construction of an optical repeating unit, which is used for any of the optical repeating units 10B1-10BN. As the construction of the optical repeating units 10B1-10BN are all identical, the following description will be given only for the optical repeating unit 10B1 and description of other optical repeating units will be omitted.
Referring to FIG. 2, the optical repeating unit 10B1 comprises a clock-extracting unit 10b1 for extracting an optical clock signal from the incoming optical signal component for the channel CH1 and further an optical re-modulating unit 10b2 supplied with the optical clock signal thus extracted. Thereby, the optical re-modulating unit 10b2 is supplied with the incoming optical signal component directly and modulates the optical clock signal extracted by the foregoing clock-extracting unit 10b1 with the incoming wavelength-division multiplexed optical signal. Thus, the optical re-modulating unit 10b2 produces an output optical signal corresponding to the incoming optical signal component with compensation for the decay of signal waveform or phase offset and further with noise elimination.
As represented in FIG. 2, the clock-extracting unit 10b1 is in fact formed of a mode-locking laser diode 10c1 that produces an optical pulse train continuously, and the extraction of the optical clock signal is achieved by synchronizing the optical pulse trains to the clock signals in the incoming optical signal component. The optical re-modulating unit 10b2 comprises, on the other hand, a laser-diode amplifier 10c2 and carries out modulation of the optical clock signal extracted by the clock-extracting unit 10b1 by the incoming optical signal.
Thus, the optical repeater 10 of the related art has an advantageous feature of having a simple construction in each of the optical repeating units 10B1-10BN and capability of reproducing the incoming wavelength-division multiplexed optical signal directly, without converting the same once to an intermediate electrical signal and re-converting the intermediate electrical signal again to the optical signal.
On the other hand, the optical repeater 10 of the related art raises a problem, when the number of the channels has increased up to 100 or more for example, in that it requires numerous and complex optical waveguides for handling a large number of channels. Thereby, such an optical repeater 10 would require an optical phase compensator for each of the optical waveguides for compensating for any optical phase offset that may be caused as a result of the use of large number of optical waveguides of complex shape and varying lengths. Naturally, the process of adjusting these optical phase compensators for each of the optical waveguides according to the optical lengths thereof becomes an extremely tedious and complex proces, and the cost of the optical signal repeater is increased inevitably.
In the event the wavelength-division multiplexed optical signal is processed directly in the optical repeater 10 of FIGS. 1 and 2 without decomposing into individual channels, there arises a problem of cross-talk between the optical channels.
Accordingly, it is a general object of the present invention to provide a novel and useful optical processor of wavelength-division multiplexed optical signals wherein the foregoing problems are eliminated.
Another object of the present invention is to provide an optical processor of wavelength-division multiplexed optical signals wherein an incoming wavelength-division multiplexed optical signal is processed without decomposing into optical signal components of individual channels.
Another object of the present invention is to provide an optical processor of a wavelength-division multiplexed optical signal comprising:
a semiconductor substrate having a first conductivity type;
a quantum semiconductor device formed on said semiconductor substrate;
a first optical waveguide formed on said semiconductor substrate in optical coupling with said quantum semiconductor device; and
a second optical waveguide formed on said semiconductor substrate in optical coupling with said quantum semiconductor device,
said quantum semiconductor device comprising:
a first cladding layer of said first conductivity type formed on said semiconductor substrate;
an undoped active layer formed on said first cladding layer;
a second cladding layer of a second conductivity type formed on said undoped active layer;
a first ohmic electrode connected electrically to said semiconductor substrate; and
a second ohmic electrode connected electrically to said second cladding layer,
said first optical waveguide making an optical coupling with said active layer, said first optical waveguide guiding a wavelength-division multiplexed optical signal incident thereto at an input end thereof to said active layer,
said second optical waveguide making an optical coupling with said active layer, said second optical waveguide guiding said wavelength-division multiplexed optical signal processed in said active layer to an output end of said second optical waveguide,
said undoped active layer comprising:
an undoped compound semiconductor layer having a bandgap smaller than any of said first and second cladding layers and a plurality of mutually separated quantum dots formed inside said undoped compound semiconductor layer with a bandgap smaller than said bandgap of said undoped compound semiconductor layer.
According to the present invention, it becomes possible to carry out processing of a wavelength-division multiplexed optical signal directly, without decomposing the same into optical signal components of individual channels, by using a non-linear optical semiconductor device that uses self-organized quantum dots appearing in a strained heteroepitaxial system.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.